1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for exposing a stimulable phosphor to a radiation to have a radiation image stored therein, scanning the stimulable phosphor by stimulating rays to cause the stimulable phosphor carrying the radiation image stored therein to emit light in proportion to the radiation energy stored, detecting the emitted light and converting it into an electric signal, and reproducing a visible image by use of the obtained electric signal. This invention particularly relates to a radiation image recording and readout apparatus in which sheets comprising the stimulable phosphor are circulated and reused for recording radiation images, and image read-out is conducted efficiently in accordance with the image recording areas in the stimulable phosphor sheets.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned by stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted by the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examination, it is disadvantageous to load a mobile X-ray diagnostic station with a number of stimulable phosphor sheets, and the amount of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, once store the radiation images of the objects in the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examinations. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining on the stimulable phosphor sheet after it is scanned by stimulating rays to read out the radiation image stored therein should be eliminated or erased by the method as described, for example, in U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599. The stimulable phosphor sheet can then be used again for radiation image recording.
Accordingly, it is desired that there be combined into a single apparatus: an image recording section for exposing each circulatable and reusable sheet comprising a stimulable phosphor to a radiation passing through an object, an image read-out section for reading out the radiation image stored in the stimulable phosphor sheet, and an erasing section for erasing the radiation energy remaining on the stimulable phosphor sheet after the read-out step for the purpose of again recording another radiation image thereon. This is because such an arrangement would make it possible to easily load the apparatus on the mobile X-ray diagnostic station to conduct medical examinations at different locations. Such an apparatus could also be easily installed in a hospital or the like. This is very advantageous in practical use.
However, when stimulable phosphor sheets are circulated and reused as described above, the size of the used stimulable phosphor sheets is limited. Therefore, a small radiation image of, for example, the palm of the hand or the diseased region may have to be recorded on a stimulable phosphor sheet of a large size adapted for recording a radiation image of, for example, the frontal chest. When a small radiation image is recorded on a unnecessarily large stimulable phosphor sheet, since the image read-out operation is conducted also for a wide marginal portion outside of the necessary image portion which should be reproduced for viewing purposes, the image read-out time becomes unnecessarily long. Further, in the case where a radiation image of a small diseased region is recorded on the stimulable phosphor sheet, there arise not only the problem that the image read-out time becomes unnecessarily long as described above but also the problem that the tissues of the human body outside the diseased region are unnecessarily exposed to radiation. Since the radiation may harm the human body, this problem must be eliminated from the viewpoint of safety. For this purpose, it becomes necessary to conduct a troublesome operation such as application of lead plates to the portions of the human body of which a radiation image is not required to be recorded in the stimulable phosphor sheet.
The problems mentioned above can be solved by preparing the circulatable and reusable stimulable phosphor sheets in various sizes and selecting the sheet size in accordance with the area of the portion for which the radiation image is to be recorded. However, in this case, it is necessary to select the sheet size and feed the stimulable phosphor sheet of the selected size each time image recording is conducted, and the image recording operation becomes complicated.